In order to lighten various optical modulation devices and display devices or to reduce power consumption required for these devices, a tendency of using liquid crystal devices for these devices has been recently increased. Of various liquid crystal devices, ferroelectric or antiferroelectric liquid crystal devices using electrooptic effect of a smectic liquid crystal phase (hereinafter referred to as "smectic liquid crystal device") have been paid attention because they have a wider view angle of field and a higher response speed as compared with TN liquid crystal devices. Particularly because the antiferroelectric crystal devices are excellent in impact resistance and almost free from a problem of printing a liquid crystal material on the inner surface of a liquid crystal cell, they have been recently paid much attention.
The smectic liquid crystal device includes a liquid crystal cell in which a liquid crystal material capable of exhibiting a smectic liquid crystal phase (hereinafter referred to as "smectic liquid crystal material") with a thickness of not more than several .mu.m is filled between electrodes provided on a pair of substrates, and also includes, if necessary, various polarization control means. For conducting optical modulation with a sufficiently high contrast using the device, a layer of the smectic liquid crystal material in the liquid crystal cell is desired to be uniformly orientated so that the layer normal directions of the smectic liquid crystal (normal directions against the smectic liquid crystal layer) shown in FIGS. 3-A, 3-B and 3-C are almost the same as each other. In these figures, the liquid crystal molecules are orientated in such a manner that each liquid crystal molecule has a tilt angle .THETA., to form a liquid crystal layer. The layer normal shown in FIG. 3 is parallel with the electrode surface of the liquid crystal cell. In FIG. 3-A, a liquid crystal layer formed from liquid crystal molecules whose every spontaneous polarization is in the upward direction and a liquid crystal layer formed from liquid crystal molecules whose every spontaneous polarization is in the downward direction are layered one upon another. In FIG. 3-B, liquid crystal layers each formed from liquid crystal molecules whose every spontaneous polarization is in the upward direction are layered one upon another. In FIG. 3-C, liquid crystal layers each formed from liquid crystal molecules whose every spontaneous polarization is in the downward direction are laminated one upon another.
Particularly in the case of optical switches for optical communication, optical shutters for stereoscopic image display and spatial optical modulators used for image display, image processing and optical operation, a contrast of at least about 100 is necessary, so that it is required that not only the smectic liquid crystal layer is uniformly orientated all over the device surface but also occurrence of various defects in the smectic liquid crystal layer is inhibited.
For making the layer normal directions of the smectic liquid crystal the same as each other, there is conventionally known a method of first forming an orientation film made of a polymer thin film such as a polyimide thin film on the interface between the substrate and the liquid crystal material and then subjecting the orientation film surface which is to be brought into contact with the liquid crystal material to a rubbing treatment. The term "rubbing treatment" used herein means a treatment of rubbing the surface of the orientation film formed on the substrate in a given direction with a cloth or the like.
When the liquid crystal material is filled between the orientation films whose rubbing directions are parallel with each other, however, plural domains having layer normal directions of the smectic liquid crystal different from those of their adjacent domains are often formed. Especially in the case where the layer normal directions of the smectic liquid crystal are strikingly different with domains, if electric voltage is applied to the liquid crystal cell to change the orientation state of the smectic liquid crystal layer inside the liquid crystal cell, light leaking takes place because the extinction positions of the smectic liquid crystal layer differ with domains. Therefore, the liquid crystal device provided with such a liquid crystal cell as mentioned above has a problem that the contrast between brightness and darkness of the output lights before and after optical modulation is conducted by this liquid crystal device becomes smaller as compared with the case of using a liquid crystal device in which the layer normal directions of the smectic liquid crystal present inside the liquid crystal cell are the same as each other. That is, a sufficiently high contrast cannot be obtained by the use of a ferroelectric or antiferroelectric liquid crystal device provided with a liquid crystal cell in which a liquid crystal material is filled between such orientation films as have rubbing directions parallel with each other.
For solving this problem, particularly in the antiferroelectric liquid crystal device, there have been proposed methods of shifting the rubbing direction against the orientation film formed on each of the upper and lower substrates. For example, Japanese Patent Laid-Open Publication No. 371925/1992 proposes a method of shifting the rubbing direction by the sum (.THETA..sub.1 +.THETA..sub.2) of the angle .THETA..sub.1 which is formed by the rubbing direction and the layer normal of the liquid crystal in a smectic A phase on the surface of the orientation film provided on the upper substrate and the angle .THETA..sub.2 which is formed by the rubbing direction and the layer normal of the liquid crystal in a smectic A phase on the surface of the orientation film provided on the lower substrate. Japanese Patent Laid-Open Publication No. 3676/1994 proposes a method of shifting the rubbing direction by a given angle.
However, the method proposed in Japanese Patent Laid-Open Publication No. 371925/1992 is not applicable because the angle between the rubbing directions on the surfaces of the orientation films formed on the upper and lower substrates cannot be determined in the case of a liquid crystal material whose layer normal against the rubbing direction is not decided univocally or a liquid crystal material which cannot be in a smectic A phase.
Also in the method proposed in Japanese Patent Laid-Open Publication No. 3676/1994, there is a problem that the preferred range of the angle to obtain sufficiently uniform orientation is not always clear.
In the above-mentioned conventional methods, moreover, stripe defects induced by application of an electric field cannot be inhibited, and hence when switching between the ferroelectric states, which is brought about by applying electric voltage to the antiferroelectric liquid crystal device, is used for the optical modulation, a sufficiently high contrast can be hardly obtained only by shifting the rubbing direction of the surface of the orientation film formed on each of the upper and lower substrates by application of an electric field.